Probable Autoimmune Encephalitis with Paraneoplastic Features Presenting as Rapidly Progressive Dementia in an Older Adult

An older man in his early seventies presented with rapidly progressive cognitive and functional decline characterized by spatial disorientation, progressive speech impairment culminating in mutism, paratonia, and a frontal gait disturbance. Initial magnetic resonance imaging showed only small-vessel ischemic changes, whereas fluorodeoxyglucose (FDG) positron emission tomography–computed tomography (PET-CT) demonstrated diffuse cortical hypometabolism with relative mesial temporal hypermetabolism. Serum testing revealed a borderline antititin antibody, and systemic PET-CT imaging showed FDG-avid colonic and cervical lymph node lesions, raising concern for a possible paraneoplastic process. Histological confirmation was not feasible due to significant frailty and cognitive impairment. High-dose corticosteroid therapy resulted in no neurological improvement. Overall, the clinical course, metabolic imaging findings, and antibody profile supported a diagnosis of probable autoimmune encephalitis according to Graus criteria, with paraneoplastic features considered based on systemic imaging abnormalities. This case highlights the diagnostic challenges of rapidly progressive dementia in older adults.